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Graphene or large PAH? June 2, 2008

Posted by fetzthechemist in Speculation.
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Graphene…..it is used by materials svientists as if it were a discreet chemical structure, a certain type of fused ring aromatic hydrocarbon. But is this really a good model?

Graphite has interesting electrical properties. Its ionization potential (IP) and electron affinity (EA) are the same value, 4.39 electron volts. All large PAHs that have had these values determined have higher IPs and lower EAs, a gap between the two values. This is ascribed to a real PAH having peripheral hydrogens, a less than infinite symmetry, and to edge discontinuities because the borders of the aromatic rings for bulges and indentations.

If you look at optical properties of large PAHs of up to at least about 30 rings and eighty ring carbons, they are colored red or bright orange and have discreet UV and fluorescence spectra. They look like bigger PAHs, not like small graphitic materials.

As larger and larger PAHs have been synthesized and examined, the theory of where the large aromatic core of a PAH will look like graphite keeps getting increased. In the 1980s and 1990s, it was estimated to be thirty or forty ring carbons. Now it is variously estimated at over a hundred.

Graphene materials in the literature are often produced as complex mixtures resulting from cleaving and splitting off parts of a graphitic sheet. Could the interesting properties of graphene be due more to its heterogeneity than to its aromatic core sizes?



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